This invention relates to a method for preparing new unsaturated polyester resins containing high amounts of dicyclopentadiene, the resultant resin product and also to curable liquid compositions containing said resins together with a monomeric copolymerizable component.
Cyclic structures in the alkyd portion of a polyester normally increase compatibility with styrene and provide rigidity and toughness in the cured resin. Low-cost phthalic anhydride is the most widely used cyclic polyester intermediate. Substitution of other cyclic intermediates for phthalic anhydride in a polyester formulation, however, often results in an improvement of one or more resin properties; for example, isophthalic acid polyesters have better resilience and corrosion resistance, and polyesters derived from certain bisphenol A derivatives have very superior corrosion resistance.
Dicyclopentadiene is a well-known, readily available raw material available from the petroleum industry.
Although dicyclopentadiene exists both as an endo and exo isomer, the commercial product is a liquid consisting principally of the endo form and contains a minimum of 90 weight percent available cyclopentadiene. Since dicyclopentadiene is very susceptible to peroxide formation upon contact with air, the commercial product is usually inhibited with p-tertiary butyl catechol.
Dicyclopentadiene is used principally as a source of cyclopentadiene, a highly reactive monomer, and being a conjugated diene, it readily undergoes the Diels-Alder reaction to form cyclohexene derivatives having a bridged ring or endomethylene group.
The following methods for incorporating dicyclopentadiene into a polyester are known. The anhydride method has been known wherein the preformed Diels-Alder adduct of cyclopentadiene and maleic anhydride is reacted with further amounts of maleic anhydride and a glycol in a polyesterification reaction at 200.degree. C. Another method involves reacting a mixture of maleic anhydride, dicyclopentadiene and a glycol at 160.degree. C.-180.degree. C. followed by polyesterifying at 200.degree. C.
Yet another known method consists of pre-reacting maleic anhydride and glycol at 160.degree. C. to obtain an esteracid prior to reaction with dicyclopentadiene at 170.degree.-175.degree. C. and polyesterification at 200.degree. C. A further known method involves polyesterification of maleic anhydride and a glycol at 200.degree. C. followed by subsequent reaction with dicyclopentadiene at 160.degree.-170.degree. C.
Investigators have found using the above prior art methods that essentially two structurally different polyesters are formed, one containing groups (I) resulting from Diels-Alder reaction whereas the other contains ester and ether groups (II) resulting from acid or hydroxyl addition to one of the dicyclopentadiene double bonds. ##STR1##
The polyesters of structure (II) have a higher activity relative to those of structure (I).
It has also been known to prepare thermosetting polyester compositions by mixing together a vinyl monomer such as, for example, styrene and an unsaturated polyester resin such as the prior art resins discussed above or those obtained by condensing maleic anhydride with a dihydric alcohol, such as ethylene glycol. The constituents of such compositions upon gentle heating and particularly in the presence of a peroxide or azo catalyst readily copolymerize or crosslink to infusible products that are substantially insoluble in many common organic solvents.